1. Field of the Invention
The present invention relates generally to a piezoelectric resonator utilizing a thickness shear vibration mode and a method of fabricating the same, and more particularly, to a piezoelectric resonator having a structure in which the thickness of a product having the resonator is independent of the resonance frequency, and a method of fabricating the same.
2. Description of the Prior Art
Referring to FIGS. 11 and 12, one example of a conventional piezoelectric resonator will be described. A piezoelectric resonator 1 shown in FIG. 11 is constructed using a piezoelectric resonance element 2 of an energy-trapped type utilizing a thickness shear vibration mode, spacers 3 and 4, and cover plates or sheets 5 and 6, as shown in FIG. 12.
The piezoelectric resonance element 2 has a structure in which electrodes 2b and 2c are respectively formed on both major surfaces of a piezoelectric substrate 2a. The electrodes 2b and 2c are opposed to each other while being separated by the piezoelectric substrate 2a in the center of the piezoelectric substrate 2a, thereby to form a vibrating portion of an energy-trapped type utilizing a thickness shear vibrating mode in a region where the electrodes 2b and 2c are opposed to each other.
The spacers 3 and 4 are respectively arranged in side parts of the piezoelectric resonance element 2. The spacers 3 and 4 are constituted by rectangular insulating plates and have cutout portions 3a and 4a formed by cutting away parts of side surfaces of the insulating plates, respectively. The cutout portions 3a and 4a are provided so as to form gaps for not preventing the vibration of the above described vibrating portion.
In the fabrication of the piezoelectric resonator, the spacers 3 and 4 are first respectively fastened to both side surfaces of the piezoelectric resonance element 2 and then, the cover sheets 5 and 6 are respectively affixed to the piezoelectric resonance element 2 and the spacers 3 and 4 so as to cover both major surfaces of the piezoelectric resonance element 2 and the spacers 3 and 4. The cover sheets 5 and 6 are respectively constituted by rectangular insulating members having notches 5a and 5b and 6a and 6b. The notches 5b and 6a are respectively provided so as to expose the electrodes 2b and 2c in the piezoelectric resonance element 2 to the exterior (see FIG. 11) to make it easy to make electrical connection to the exterior.
Reference numerals 7 and 8 denote adhesive layers. As shown in FIG. 12, the adhesive layer 8 is provided in a peripheral region excluding a central region 6c on the upper surface of the cover sheet 6. This is for forming a gap so that the thickness of the adhesive layer 8 does not prevent the vibration of the vibrating portion in the piezoelectric resonance element 2 after lamination. Similarly, the adhesive layer 7 is provided in a peripheral region excluding a central region on the lower surface of the cover sheet 5.
In the piezoelectric resonator 1 shown in FIG. 11, the thickness t of the piezoelectric resonance element 2 utilizing a thickness shear vibration mode is determined by a desired resonance frequency. For example, when the resonance frequency is 3.58 MHz, the thickness t must be generally about 0.35 mm. Consequently, when it is desired to limit the entire thickness T of the piezoelectric resonator 1 to be, for example, 0.5 mm or less, the total of the thickness of the adhesive layers 7 and 8 and the cover sheets 5 and 6 must be 0.15 mm or less. As a result, the thicknesses of the cover sheets 5 and 6 must be substantially decreased, thereby to make it impossible to obtain a piezoelectric resonator having considerable mechanical strength.